1. Field of the Invention
The present invention relates generally to global navigation satellite system (GNSS) control systems and methods for mobile, fixed-course equipment for irrigation and related applications.
2. Description of the Related Art
GNSS guidance and control are widely used for vehicle and personal navigation and a variety of other uses involving precision location in geodesic reference systems. GNSS, which includes the Global Positioning System (GPS) and other satellite-based positioning systems, has progressed to sub-centimeter accuracy with known correction techniques, including a number of commercial Satellite Based Augmentation Systems (SBASs).
Agricultural equipment represents a significant market for GNSS control systems. Various aspects of agricultural equipment guidance can benefit from GNSS technology, including tillage, planting, spraying (e.g., applying fertilizer, herbicides, pesticides, etc.) and harvesting. In arid regions, mechanized irrigation techniques and equipment have greatly increased crop production and correspondingly made vast tracts of previously barren land viable for productive agriculture, thus enhancing its value and crop yields. Broadly speaking, mechanized irrigation involves pumping water from a source and spraying it on crops and/or fields. Although sprayer vehicles are extensively used and widely available, their operation tends to be relatively expensive due to operating costs such as labor and fuel, and capital costs of vehicles. Therefore, automatic irrigation systems tapping into subterranean aquifers are widely used and cover large areas relatively cost-effectively. Typical large-scale irrigation uses distribution piping carried on self-propelled support towers and supplying multiple spray nozzles, which pass over crops and distribute water evenly thereon. Self-propelled irrigation equipment is constructed for linear operation, e.g., along guide paths, and for center-pivot operation, which produces circular irrigation patterns.
Such irrigation equipment tends to move relatively slowly and operate relatively automatically, and is thus ideal for autonomous guidance and control. Previous methods of monitoring and guiding irrigation devices include positive position encoders located at the center pivot for monitoring angular or rotational orientation of the irrigation boom with respect to the center pivot. Another method included buried wires and sensors, which were detectable by equipment mounted on the rotating booms.
Global navigation satellite systems (GNSS), including global positioning systems (GPS), have also been used for center pivot irrigation monitoring and guidance. For example, U.S. Pat. No. 6,095,439 discloses a corner irrigation system including a GPS guidance system. Field corners, which would otherwise fall outside a circular coverage pattern, are accommodated by an extension boom, which is pivotable with respect to a main boom and swings out into the field corners under GPS guidance. However, previous GPS guidance and control systems for agricultural irrigation have tended to be relatively expensive, complex, inaccurate and/or susceptible to other deficiencies and disadvantages.
It is known in the art that by using GPS satellites' carrier phase transmissions, and possibly carrier phase signal components from base reference stations or Space Based Augmentation Systems (SBAS) satellites, including Wide Area Augmentation System (WAAS) (U.S.), and similar systems such as EGNOS (European Union) and MSAS (Japan), a position may readily be determined to within millimeters. When accomplished with two antennas at a fixed spacing, an angular rotation may be computed using the position differences. In an exemplary embodiment, two antennas placed in the horizontal plane may be employed to compute a heading (rotation about a vertical axis) from a position displacement. Heading information, combined with position, either differentially corrected (DGPS) or carrier phase corrected (RTK), provides the feedback information desired for a proper control of the vehicle direction.
Another benefit achieved by incorporating a GPS-based heading sensor is the elimination or reduction of drift and biases resultant from a gyro-only or other inertial sensor approach. Yet another advantage is that heading may be computed while movable equipment is stopped or moving slowly, which is not possible in a single-antenna, GPS-based approach that requires a velocity vector to derive a heading. Yet another advantage is independence from a host vehicle's sensors or additional external sensors. Thus, such a system is readily maintained as equipment-independent and may be moved from one vehicle to another with minimal effort. Yet another exemplary embodiment of the sensor employs Global Navigation Satellite System (GNSS) sensors and measurements to provide accurate, reliable positioning information. GNSS sensors include, but are not limited to GPS, Global Navigation System (GLONAS), Wide Area Augmentation System (WAAS) and the like, as well as combinations including at least one of the foregoing.
An example of a GNSS is the Global Positioning System (GPS) established by the United States government, which employs a constellation of 24 or more satellites in well-defined orbits at an altitude of approximately 26,500 km. These satellites continually transmit microwave L-band radio signals in two frequency bands, centered at 1575.42 MHz and 1227.6 MHz, denoted as L1 and L2 respectively. These signals include timing patterns relative to the satellite's onboard precision clock (which is kept synchronized by a ground station) as well as a navigation message giving the precise orbital positions of the satellites, an ionosphere model and other useful information. GPS receivers process the radio signals, computing ranges to the GPS satellites, and by triangulating these ranges, the GPS receiver determines its position and its internal clock error.
In standalone GPS systems that determine a receiver's antenna position coordinates without reference to a nearby reference receiver, the process of position determination is subject to errors from a number of sources. These include errors in the GPS satellite's clock reference, the location of the orbiting satellite, ionosphere induced propagation delay errors, and troposphere refraction errors.
To overcome these positioning errors of standalone GPS systems, many positioning applications have made use of data from multiple GPS receivers. Typically, in such applications, a reference receiver, located at a reference site having known coordinates, receives the GPS satellite signals simultaneously with the receipt of signals by a remote receiver. Depending on the separation distance between the two GPS receivers, many of the errors mentioned above will affect the satellite signals equally for the two receivers. By taking the difference between signals received both at the reference site and the remote location, these errors are effectively eliminated. This facilitates an accurate determination of the remote receiver's coordinates relative to the reference receiver's coordinates.
The technique of differencing signals from two or more GPS receivers to improve accuracy is known as differential GPS (DGPS). Differential GPS is well known and exhibits many forms. In all forms of DGPS, the positions obtained by the end user's remote receiver are relative to the position(s) of the reference receiver(s). GPS applications have been improved and enhanced by employing a broader array of satellites such as GNSS and WAAS. For example, see commonly assigned U.S. Pat. No. 6,469,663 to Whitehead et al. titled Method and System for GPS and WAAS Carrier Phase Measurements for Relative Positioning, dated Oct. 22, 2002, the disclosures of which are incorporated by reference herein in their entirety. Additionally, multiple receiver DGPS has been enhanced by utilizing a single receiver to perform differential corrections. For example, see commonly assigned U.S. Pat. No. 6,397,147 to Whitehead titled Relative GPS Positioning Using A Single GPS Receiver With Internally Generated Differential Correction Terms, dated May 28, 2002 the disclosures of which are incorporated by reference herein in their entireties.
Heretofore there has not been available a GNSS control system for agricultural irrigation and related applications with the advantages and features of the present invention.